With the advent of the energy crisis, there has been considerable interest in the recovery of energy which might otherwise be lost, such as the heat contained in the fluid discharged from a dishwasher or the like. In my copending application Ser. No. 920,660 filed June 30 1978, now abandoned, there is disclosed a method and apparatus for recovery of waste energy, as from a dishwasher. An important part of this method and apparatus is a liquid to liquid heat exchanger, through which waste water, such as discharged from a dishwasher, is passed in order to heat fresh water, such as the incoming feed water for a hot water heater. Such heat exchange avoids the heat being lost in the used wash water through its being discarded through a conventional drain. However, the temperature differential between the warmer liquid, such as waste water, and the cooler liquid, such as hot water heater makeup water, may be relatively slight, so that the heat exchanger must necessarily be highly efficient to transfer the maximum amount of heat from the warmer liquid to the cooler liquid. An effective and efficient heat exchanger is also disclosed in the aforesaid application Ser. No. 920,660, consisting of sets of multiple tubes, such as three, in spaced, spiral relation between thin cylinders. For more effective heat transfer, the warmer liquid is circulated through the tubes, while the cooler liquid is forced to follow a circuitous path between the turns of the respective spiral coils, to achieve maximum contact time of the cooler liquid with the exterior walls of the coils. In such a heat exchanger, the tubes of the coils abut to force the liquid to be heated to follow such a spiral path. However, in such a heat exchanger, if a leak occurs in any tube of the coil, the contaminated water will leak out into the fresh water being heated. Since a hot water heater, in many instances, not only supplies a dishwasher but also wash basins, certain plumbing codes reduce considerably the use of such a heat exchanger. Thus, it is desirable to provide an efficient and effective heat exchanger in which the warm liquid and cool liquid each flow in separate tubes.
In U.S. Pat. No. 1,965,553, a series of rectangular or triangular tubes in concentric cylindrical coils are wound in spiral relation from the inside out, so that heat will be transferred between different liquids, such as a refrigerant in one tube forming alternating turns of a coil, with other liquids in different tubes, such as liquids to be cooled, in the alternating turns of different coils. The heat exchange between the tubes is effected by metal to metal contact of the tubes. When the tubes are rectangular, two sides of each tube are in metal to metal contact with the abutting sides of adjoining turns of the same coil, while the top and bottom of each turn is in metal to metal contact with the bottom and top, respectively, of the turns of adjacent inner and outer coils, When the tubes are triangular in cross section, the apices of the triangles point outwardly in a toothed arrangement in alternating cylindrical coils with the apices of the tubes pointing inwardly to intersect the preceding apices. Thus, there is theoretically produced metal to metal contact of each side of a triangular tube with the sides of the triangles in adjacent turns of the same coil. Also, the base of each triangular tube in the respective coils will face alternately inwardly and outwardly, so that the base of a triangle in one coil will be in metal to metal contact with the base of a triangle of a tube turn of the next inner or outer row. Although the metal to metal contact of the rectangular or triangular tubes is conducive to heat exchange, the turns, due to deformation, misalignment, twisting or other conditions, may reduce the heat exchanged. There are also numerous problems involved in the construction of such a heat exchanger, particularly since both the rectangular and trianglar tubes must always be kept in essentially perfect alignment with the turn whose side, top or bottom walls are to be engaged. Since tubing is generally wound in a coil, the unwinding and rewinding in a specific relationship often produces a tendency for the tube to twist, such twisting often causing difficulty in rewinding. In addition, the cost of producing rectangular or triangular tubes is much greater than that of circular or cylindrical tubes, with the result that the cost of a heat exchanger having rectangular or triangular tubes may exceed the value of the energy which might be recovered within a reasonable time.
It is generally known to use a heat conductive mastic, i.e. a resin having heat conducting properties to improve heat conductivity between metals, as in U.S. Pat. Nos. 3,554,275 and 3,666,006 and British patent No. 938,372. Also, in U.S. Pat. No. 4,123,837, the use of a heat conductive mastic to improve heat transfer between a pipe and a much larger conduit or vessel is proposed, with an angular cover for the mastic to retain the mastic in position. It is also suggested that the mastic may be applied to the larger vessel and the heating pipe pushed into the mastic. Nevertheless, these prior patents do not disclose the novel features of the present invention.
It will also be noted that a coil can be coated by dipping in a liquid, such operation normally being used for waterproofing or insulation purposes. However, the heat conductive mastic must be heated to a relatively high temperature before it becomes a liquid, and even a dipping operation will not insure that the interstices between all the coil turns, particularly those deep within the coils, will be filled.
Among the objects of this invention are to provide a novel method of producing a multiple coil, multiple tube liquid to liquid heat exchanger; to provide such a method which is particularly adapted to be utilized with tubing having a circular cross section; to provide such a method which enables a heat conductive mastic to transfer heat between a warmer liquid in one or more turns of a coil and a cooler liquid in one or more turns of the same or adjacent coils; to provide such a method which will produce an effective contact of mastic with essentially the entire surface of each tube for the length of each coil to insure that the mastic will fill the spaces between adjacent turns of each coil, as well as to insure that the mastic will fill the spaces between the turns of successive coils; to provide such a method which will insure that the mastic will remain in essentially compressed position between the turns of the respective coils; to provide such a method which will be simple and effective in operation; and to provide such a method which may be carried out without undue expense.